This invention relates generally to electrodes for monitoring cortical electrical activity and, in particular, electrode configurations and implantable devices for such electrical monitoring. The electrodes and implants are especially useful in monitoring cortical electrical activity to locate or define cortical epileptogenic foci.
Epilepsy affects approximately 2 million people in the United States and frequently begins during childhood. Although most cases of epilepsy are managed with medicines, some are best treated by locating and removing a specific lesion. These surgical procedures require detailed mapping to locate functional brain regions as well as the focus of epileptic activity. Epileptic foci may include mesial temporal sclerosis, focal cortical dysplasia, or other congenital neural development defects, or tumors. To evaluate these epilepsies, a common method has involved a highly invasive monitoring approach in which one or more multi-contact electrodes are implanted into the cortex, typically penetrating to the hippocampal depth. A problem with these deep penetrating electrodes is that the data obtained is limited to the local region in which they are placed. Furthermore, they require puncture of the brain surface for insertion. As a result of these limitations, the ability of such penetrating electrodes to study epilepsy outside their local region is sub-optimal.
In epilepsy outside of the temporal lobes, subdural electrode arrays are often employed and generally regarded as to be superior to intracortical penetrating electrodes. Subdural electrode arrays allow better localization of epileptogenic foci that are cortical in origin, and also allow mapping of brain tissue functions by the use of extra-operative stimulation. Due to the unpredictability of seizures, an extended period of time for recording of interictal and ictal activity is often needed to allow the investigator to map the epileptic focus prior to surgical resection. Long term monitoring of cortical electrical activity and in particular, long term monitoring via electrodes on the cortical surface, can yield a broader range of data regarding a seizure foci and the surrounding tissue. In addition, proper use of subdural electrodes causes less trauma and, hence, lower morbidity, in comparison to the use of intracortical penetrating electrodes with a similar capability of identifying epileptic foci.
Surgical removal of epileptogenic brain tissue is indicated for treatment of many epileptic seizure disorders. The optimal surgical outcome for the treatment of intractable seizures is only achieved following specific localization of the epileptic focus. This requires proper insertion and an understanding of the exact position of the subdural electrodes to obtain a proper interpretation of the recordings taken from the electrode contacts. Errors in the understanding of the position of the subdural electrodes can prevent accurate location of the epileptogenic foci. Great care must also be taken during insertion to avoid bending or folding the electrode, and to minimize any shift in the electrode position after insertion. This is particularly true with respect to separate arrays of subdural strip electrodes.
The prior art has attempted to overcome these limitations using two methods to prevent any bending or folding. The first includes greater structural support within the electrode body itself. The second is to create an electrode array by placing a plurality of electrodes on a rigid structure in a grid layout. These solutions however, have led to larger, more invasive, electrodes with the concomitant increase in difficulty and danger in inserting and removing them.
While grid electrode arrays exist in the prior art, they are typically rectangular electrode arrays with the contacts mounted on a rigid material. The contacts are usually monotonously separated. Thus, prior art grid electrode arrays are not designed to follow the specific contours of an individual or a particular area of the brain and, as a result, such rigid electrode grids can introduce errors in the location of the epileptogenic foci.
In addition, large grid structures, which may be as large as 8 cm.times.8 cm, require a large scale craniotomy for both insertion and again for removal of the rigid array with the inherent dangers associated with repeated resections of the cranium.
Thus, there exists a need for better subdural electrode structures. The electrode structure that can provide position stability to accurately determine the epileptogenic foci, and also allow for easy removal upon completion of the brain monitoring, would fulfill a long-felt need in the field.